The present invention relates to an apparatus and method for emitting high power acoustic waves.
The technical field of the invention is the building of electro-acoustic transducers.
The principal application of the invention is increasing the emitting power of an underwater transducer that consists of at least one headmass and a driving assembly.
Underwater electro-acoustic transducers, and in particular piezoelectric transducers, are known. Piezoelectric transducers include a rigid, hollow cylindrical shell, which is open at both axial ends. Within and along the axis of the shell, two identical electro-acoustic motors are arranged on either side of a central countermass. The opposite ends of the countermass are surrounded by a headmass. These transducers are called "Tonpilz" double type transducers. The electro-acoustic motors can be made of two piles of aligned piezoelectric wafers. The external faces of both headmasses are located in the plane of the shell axial ends, so that they are in contact with the liquid in which the shell is immersed. The external perimeter of these headmasses is very close to the edge of the open axial ends of the shell.
Thus, these external faces emit acoustic waves in the liquid when the electro-acoustic motors are electronically excited. These transducers are used to emit low frequency acoustic waves in water and in a given direction, as disclosed in French patent application FR 2 663 181, which describes additional devices that provide increased power.
To avoid the propagation of the acoustic waves emitted by the rear faces of the headmasses inside the shell, especially when the shell is filled with liquid, which results in the acoustic waves being retransmitted in the ambient medium despite the rigidity of the shell, various devices have been used. These devices include elastic tubes which are water-tight and filled with gas and are located in the cavity filled with ambient liquid at the rear part of the headmasses. The devices are such that the frequency of the Helmholtz resonance of the cavity is close to the fundamental frequency of the axial vibrations of the vibrating assembly as disclosed, e.g., in French patent FR 2 665 998.
Thus, the problem of the resistance to the external shell pressure is transferred to the resistance of the elastic tubes, which, because they have smaller diameters, make a lighter assembly possible. Other devices, however, can be developed within the same scope of the invention.
These devices require keeping a large enough cavity behind the headmass. When an increase in transducer power is desired, the volume of the electro-acoustic motors increases, which results, on the one hand, in an elongation of the electro-acoustic motors and, on the other hand, an increase in the rigidity and coefficient of the electromechanical coupling between the motors and the headmass. However, it is then necessary to increase the external space required for the transducer as well as its weight. If this is not accomplished, on the one hand, sufficient space will not be available to provide suitable means in the central cavity such as they are described above, and on the other hand, the amount of converted power will be lower.
Moreover, even if there are no disadvantages to increasing the weight and space required, the transducer pass band is narrower and lower than for a standard transducer. As a result, the transducer does not meet the needs of the desired application.
The stated problem is to be able, starting from a transducer having at least one driving assembly and at least one headmass dependent on the driving assembly, and with a given volume, to increase its power by at least 50% while remaining in a range of emission frequencies corresponding to that of the standard transducer having the same volume.